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Magnetic Shielding Extends Thruster Life
Magnetic Shielding Extends Thruster Life

JPL researchers have shown how a new technology called magnetic shielding significantly increases the life of Hall thrusters while enabling operating voltages 100 percent higher than previously demonstrated.

Image to left: Magnetically shielded operation of the H6MS Hall thruster at JPL reduces erosion rates by 2-3 orders of magnitude, potentially enabling new NASA missions not previously possible.

NASA missions have typically relied on chemical rockets for spacecraft propulsion. However, these types of rockets have a relatively low specific impulse (a measure of how efficiently the rocket uses the mass of the propellant to produce thrust) due to their reliance on the energy stored in the chemical bonds of the propellant. Alternatively, electric rockets can use an external energy source to accelerate their propellants, leading to specific impulses typically 10 times greater. This improved "fuel efficiency" comes at a price though, since the propellants are accelerated to much higher exhaust velocities that can erode the thrusters, limiting their lifetime. Recently, a team of researchers at NASA's Jet Propulsion Laboratory, Pasadena, Calif., has demonstrated through computer simulations and laboratory experiments that a type of electric rocket called a Hall thruster can be designed to effectively eliminate erosion. This development could make electric rockets a leading candidate for future NASA missions in robotic and human spaceflight.

Hall thruster
The H6MS Hall thruster installed in a vacuum chamber at JPL prior to the first demonstration of magnetic shielding at 3,000 seconds specific impulse.
Hall thrusters typically use the inert gas xenon as propellant, which is ionized in the thruster through collisions with electrons. Magnetic fields are used to shape the electric field that accelerates these ions to high velocity at 9-18-miles per second (15-30 kilometers/s), creating thrust. Though these thrusters have been flying in Earth-orbiting missions since the 1970s, they have not been used in deep space missions because they typically last less than a year. Several potential solutions have been evaluated over the years to try to extend the life of Hall thrusters, but it has not been possible to divert the accelerated ions completely away from the thruster chamber walls. A breakthrough finally came in 2010 when the company Aerojet Rocketdyne, Redmond, Wash., completed a 10,400-hour wear test of their BPT-4000 Hall thruster. It was observed that the thruster walls began to wear out at typical rates, but after about 5,600 hours, the thruster essentially stopped eroding.

Using modeling tools developed at JPL, researchers investigated and later identified the detailed physics responsible for the BPT-4000's "zero-erosion" state. This theory came to be known as "magnetic shielding" in Hall thrusters. In this technique, magnetic fields are shaped in a manner that exploits fundamental properties of the plasma discharge in these thrusters to reduce the energy of the ions hitting the walls and lower the erosion rates by several orders of magnitude.

The theory was then successfully tested at JPL and the results were published in a January issue of Applied Physics Letters. The JPL article (Appl. Phys. Lett., 102, 023509, 2013) described the validation of the magnetic shielding theory and explained how the H6 Hall thruster, originally developed by JPL, the Air Force Research Laboratory (Edwards Air Force Base, Calif.), and the University of Michigan, Ann Arbor, Mich., was modified by JPL in tests to reduce wall erosion by 2-3 orders of magnitude. This effort not only independently verified the physics of magnetic shielding but also established further that the wear test results of Aerojet Rocketdyne's BPT-4000 were not a ground facility test effect.

"Magnetic shielding solves one of the longest-standing problems in electric propulsion: the deleterious erosion of Hall thrusters by ion bombardment, thereby allowing deep-space exploration missions that could not be undertaken in the past," said Ioannis Mikellides, who developed the physics of magnetic shielding and led the two-year research program at JPL that validated the theory by experiments.

JPL researchers are now investigating how magnetic shielding can be used to extend the performance of Hall thrusters even further to higher power densities and specific impulses. Most recently, a wear test of the magnetically shielded H6MS Hall thruster was conducted at a specific impulse of 3,000 seconds, which is 50 percent higher than the 2,000 second specific impulse of the earlier work on the H6MS and BPT-4000. The results were presented in October at the 33rd International Electric Propulsion Conference (IEPC-2013-033). The wear test successfully demonstrated the effectiveness of the magnetic shielding in reducing the wall erosion at this higher specific impulse while still maintaining high performance. This was the first time that a wear test at 3,000 seconds specific impulse had been conducted on a Hall thruster where the thruster walls did not experience high erosion rates.

"Demonstrating low erosion rates at 3,000 seconds specific impulse shatters the last major barrier to the widespread adoption of Hall thrusters on robotic and human missions for NASA," said Richard Hofer, who led the 3,000 second specific impulse experiments and is also JPL's lead for Hall thruster technology development. "With these specific impulses available, we can reach the vast majority of destinations likely to be visited in the next several decades by robotic and human explorers."

While longer wear tests still need to be conducted, the results have shown that this new magnetic shielding technique makes Hall thrusters ideal candidates for the most demanding missions, including the proposed Asteroid Redirect and Retrieval Mission as a part of NASA's new Asteroid Initiative, which aims to robotically rendezvous with, capture, and return a 23-33 foot (7-10 meter) asteroid to cis-lunar space (the area between the earth and the moon) for exploration by crew in the early 2020s.

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